The present invention relates to a presensitized printing plate for waterless planographic printing, in particular offset printing, and to a process for preparing a printing form by exposure and development of said printing plate. The printing plate of this invention comprises a layer support, a radiation-sensitive layer and an outer, ink-repellent layer of silicone rubber.
Printing plates as described above and comprising positive-working or negative-working radiation-sensitive, in particular photosensitive, layers have, for example, been disclosed by U.S. Pat. No. 3,511,178. Diazonium salt condensation products are employed as negative-working, i.e., photocurable photosensitive compounds. The exposed printing plate is developed by means of a solution in which the unexposed portions of the photosensitive layer are dissolved and the silicone rubber layer is swollen. The silicone rubber portions on top of the soluble layer portions can be removed by rubbing. The printing form obtained is ink-receptive in the bared areas of the layer support, whereas the silicone rubber layer still present in the non-exposed areas of the layer support is ink-repellent. Thus, the printing form prints a negative image of the transparency employed for exposure. Insoluble precipitation products of diazonium salts and phosphotungstic acid are used as positive-working photosensitive compounds. In this case, an intermediate layer comprising a diazonium salt/formaldehyde condensate is provided between the photosensitive layer and the silicone rubber layer, in order to anchor the latter to the photosensitive layer. This intermediate layer is crosslinked by heat when the silicone elastomer layer is cured into the silicone rubber layer and then effects the desired improved adhesion.
There are also known planographic printing plates suitable for waterless printing, wherein other layers which are rendered soluble by exposure are used. Practically all of these plates require an appropriate adhesive layer.
According to German Offenlegungsschrift No. 29 43 379 (U.S. Pat. No. 4,358,522), a photosensitive layer of an o-quinonediazide and a novolak is coated with an intermediate layer of an adhesion-promoting aminoalkyl-alkoxysilane.
German Offenlegungsschrift No. 30 45 979 (U.S. Pat. No. 4,342,820) describes a combination of a particular o-quinonediazide layer and a silicone rubber layer. To improve the adhesion between these layers, silicones or silanes can be added to either one of these layers, or an adhesive layer comprising compounds of this type can be applied between the two layers.
According to the teaching of Japanese Patent Application No. 58/60744, improved adhesion is achieved by the addition of aminosilane and tetra-isopropoxytitanium to the silicone layer or by an adhesive layer comprising organotitanium compounds.
European Patent Application No. 0,100,938 describes a process for improving the adhesion of corresponding layers by a short, slight overall exposure and treatment with a base, for example, a primary amine. An intermediate layer of aminosilanes and/or organotitanates may additionally be applied.
European Patent Application No. 154,980 describes a similar, presensitized printing plate wherein the photosensitive o-quinonediazide layer additionally comprises a coupling component which in a basic medium, such as that used for development, undergoes a coupling reaction with the non-decomposed diazo compound in the unexposed areas, whereby a relatively insoluble azo dye is formed. At the same time the silicone layer is more firmly anchored to the quinonediazide layer during the coupling reaction. Additionally an adhesive layer, for example, of aminosilanes or organotitanium compounds, may be present between the aforementioned layers.
These known negative-working printing plates for waterless offset printing which are based on diazonium salts or quinone diazides have a relatively low photosensitivity. Often the adhesion can only be improved by a complicated composition of the material and additional process steps. In most cases mixtures of water-insoluble and polar organic solvents are required to develop these plates. The predominant portion of the mixtures of water-miscible solvents and water, which can be employed in some cases, comprises organic solvents. Due to this high solvent content special precautionary measures must be taken in order to prevent excessive pollution of the waste water and exhaust air.
Positive-working photosensitive compositions based on compounds which in photolytic reactions form an acid and compounds which are split by acid, for example, orthocarboxylic acid esters and acetals, have also been disclosed. These compositions can be used in the production of photosensitive layers (e.g., for printing plates) whose photosensitivity is considerably increased over that of naphthoquinonediazide layers. Compositions of this type are, for example, described in German Patent Nos. 26 10 842 (U.S. Pat. No. 4,101,323) and 27 18 254 (U.S. Pat. No. 4,247,611) and in European Patent Application No. 0,022,571 (U.S. Pat. No. 4,311,782).
Related application Ser. No. 087,677, filed Aug. 20, 1987 (German Application No. P 36 28 719.9), assigned to the assignee of the present invention and filed concurrently with this application, discloses a presensitized printing plate comprising an amorphous silicic acid between a radiation-sensitive layer and an ink-repellent silicone layer. This application is incorporated herein by reference.